Method for producing a secondary cell having flat wound electrode body

ABSTRACT

A method for producing a secondary cell having a flat wound electrode body that inhibits the bending of the electrode board caused by charging and discharging and inhibits resulting swelling of the cell and deterioration of cycle characteristics is provided. The method has the steps of: winding, with a winding core, a positive electrode board, a negative electrode board, and a separator provided between the positive and negative electrode boards, and fixing the winding end, thereby preparing an approximately cylindrical electrode body; after the step of preparing the electrode body, deforming the electrode body into a shape with an approximately oval cross section by pressing the approximately cylindrical electrode body from a direction perpendicular to the winding axis, and rotating the deformed electrode body in the winding direction, thereby relaxing the winding state; and after the relaxation steps, pressing the electrode body into the flat wound electrode body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.11/269,880, filed on Nov. 9, 2005, currently pending, which claims thebenefit of priority from the prior Japanese Patent Application Nos.2004-329614, filed on Nov. 12, 2004 and 2005-312913 filed on Oct. 27,2005, the entire contents of which are incorporated herein byreferences.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a method for producing a secondary cellhaving a flat wound electrode body that inhibits the bending of theelectrode board caused by charging and discharging, and resultingswelling of the cell and deterioration of cycle characteristics.

2) Description of the Related Art

In recent years, there has been a rapid reduction in the size and weightof mobile information terminals such as mobile phones, notebook personalcomputers, and PDAs. Secondary cells and batteries, which are thedriving power sources of such terminals, are required to be easilymounted inside the mobile information terminals, and easy to exert alarge amount of current from.

Non-aqueous electrolyte secondary cells such as lithium-ion secondarycells, which have square outer casings or laminate outer casings inwhich flat wound electrode bodies are housed, are easily mounted insidemobile information terminals and easy to exert a large amount of currentfrom because of a large area of facing between the positive and negativeelectrodes. Thus, such non-aqueous electrolyte secondary cells areuseful as the driving power sources of mobile information terminals.

In such non-aqueous electrolyte secondary cells, the positive andnegative electrode boards repeat swelling and contraction because ofintercalation and deintercalation of lithium ions through charging anddischarging. The adhesive (adhesive tape) that fixes the winding end ofthe flat wound electrode body restricts the swelling and contraction ofthe electrode boards. As shown in FIG. 8, this causes the swelling ofthe electrode boards toward the inside of the flat wound electrode body,which in turn causes bending of the electrode boards. The bending of theelectrode boards has presented the problem of increasing the thicknessof the cell. In addition, at the bent portion, a gap occurs between thepositive electrode board and negative electrode board. At this gap thesmooth movement of lithium ions are interrupted, presenting the problemof increased deterioration of cycle characteristics.

In order to solve these problems, the following attempt is being made.By using a winding core having a flat (oval, polygonal, and the like)cross section, and by reducing the tension applied on the electrodeboards at the time of winding, a space is provided between the electrodeboards of the flat wound electrode body so that the space adsorbs theswelling of the electrode boards, thus attempting to inhibit the bendingof the electrode boards. However, since the cross section of the windingcore is flat, there is a problem that the speed of winding needs to bereduced compared with the case of a circular cross section for thewinding core. In addition, since the tension applied on the electrodeboards is small, there is a problem of a reduction in the quality ofwinding (the electrode boards and separator are easily displaced whenwound).

Techniques related to the non-aqueous electrolyte secondary cell areproposed in patent documents 1 to 4, and the outlines are as follows.

Patent Document 1: Japanese Patent Application Publication No.2003-157888 (claims and paragraphs 0005-0009).

Patent Document 2: Japanese Patent Application Publication No.2003-338307 (claims and paragraphs 0013-0029).

Patent Document 3: Japanese Patent Application Publication No. 11-121044(paragraphs 0004 and 0005).

Patent Document 4: Japanese Patent Application Publication No. 11-176476(paragraphs 0004-0009).

(i) Patent documents 1 and 2 propose the following technique. In orderto absorb the deforming stress of the electrode boards, a space portionis provided between two neighboring strip laminates of a group of flatwound electrode boards (flat wound electrode body). The two neighboringstrip laminates are on the long axis line of a cross sectionperpendicular to the winding axis of the group of flat wound electrodeboards.

According to this technique, since the space portion absorbs thedeforming stress of the electrode boards, bending caused by swelling andcontraction of the electrode boards is eliminated. However, with thistechnique, a spacer is used when preparing a cylindrical electrode body,and therefore, after pressing the body flat, the step of removing thespacer is required. It is difficult, however, to wind the electrodeboards with the spacer on, presenting the problem of poor productivity.

(ii) Patent document 3 proposes the technique of using, as the adhesivefor fixing the winding end of the cell element (wound electrode body), amaterial that swells, dissolves, or decomposes after contact with theelectrolytic solution.

According to this technique, since the swelling, dissolution, ordecomposition of the adhesive provides relaxation to the winding of thecell element, the stress applied on the constituents of the element isalleviated, thereby preventing associated deterioration of cellcharacteristics.

However, with this technique, the relaxed winding of the cell elementcauses a poor state of facing between the positive and negativeelectrode boards in the cell element. This interrupts the smoothprogress of the charge-discharge reaction, presenting the problem ofdeteriorating cell characteristics such as cycle characteristics.

(iii) Patent document 4 proposes the technique of using, as the adhesivefor fixing the cell element (wound electrode body), a material that,while substantially not dissolving or decomposing in the electrolyticsolution, lowers its function of fixing the cell element after contactwith the electrolytic solution.

According to this technique, since the function of fixing the cellelement is lowered after the adhesive comes in contact with theelectrolytic solution, and the winding of the cell element is therebyrelaxed, the stress applied on the constituents of the element isalleviated, thereby preventing associated deterioration of cellcharacteristics. Also, this prevents contamination of the electrolyticsolution which is caused by dissolution of the adhesive in theelectrolytic solution. However, with this technique, similarly to thetechnique of patent document 3, there is a problem that cellcharacteristics such as cycle characteristics deteriorates.

SUMMARY OF THE INVENTION

In view of the foregoing and other problems, it is an object of thepresent invention to provide a cell that inhibits the bending of theelectrode board caused by charging and discharging.

(1) In order to accomplish the above and other objects, a firstembodiment of the present invention is configured as follows.

A method for producing a secondary cell having a flat wound electrodebody, the method comprising the steps of; winding, by using a windingcore, a positive electrode board, a negative electrode board, and aseparator provided between the positive and negative electrode boards,and fixing a winding end, thereby preparing an approximately cylindricalelectrode body; after the step of preparing the approximatelycylindrical electrode body, deforming the approximately cylindricalelectrode body into a shape with an oval cross section by pressing theapproximately cylindrical electrode body from a direction perpendicularto an axis of winding, and rotating the deformed electrode body in thesame direction as the direction of winding, thereby relaxing a state ofwinding; and after the relaxation step, pressing the electrode body intothe flat wound electrode body.

This method includes deforming the approximately cylindrical electrodebody with a fixed winding end into a shape with an oval cross section bypressing the body from a direction perpendicular to the axis of winding,and rotating the deformed electrode body in the same direction as thedirection of winding. By this step, the state of winding of theelectrode boards is relaxed without undermining the fixed state of thewinding end of the electrode body. After this step, by pressing thedeformed electrode body into a flat wound electrode body, the relaxation(clearance) is moved to the vicinity of the corner portion of the flatwound electrode body. When the electrode boards swell throughcharge-discharge cycling, they are deformed in a direction that therelaxation is filled, thereby preventing the bending of the electrodeboards. This inhibits an increase in the thickness of the cell and theoccurrence of a gap between the positive and negative electrode boards,even when charge-discharge cycling is repeated, thereby reducingdeterioration of cycle characteristics.

In addition, since the fixed state of the winding end of the electrodebody is not undermined, the state of facing between the positive andnegative electrode boards is not undermined, either.

In addition, this method is simple in that after preparing anapproximately cylindrical electrode body in a usual manner involvingwinding, the electrode body is rotated in the same direction as thedirection of winding, and thus excellent in productivity.

The term “approximately cylindrical” is intended to include, as well asthe shape of a round cylinder, the shape of an oval cylinder in whichthe ratio of the long axis to the short axis is within 1.5, and othershapes similar to the foregoing. As for the winding core used forwinding, other than one with a round cross section, one with such across section can be used that the ratio of the long axis to the shortaxis is within 1.5.

In the above method, the relaxation step may be carried out afterremoving the winding core.

In the above method, the relaxation step may be carried out afterreducing the diameter of the winding core.

In the relaxation step, where the electrode body is deformed into ashape with an oval cross section by being pressed from a directionperpendicular to the axis of winding, the winding core used for windingfunctions to interrupt such deformation. However, in the above twofeatures, such a problem does not occur because a hollow or hole is leftat the portion of the winding core used to winding.

To reduce the diameter of the winding core, it is possible to remove thewinding core after winding and to place a winding core with a smallerdiameter, or to use a winding core having a hollow and to, afterwinding, deform the winding core to fill the hollow or hole, therebyreducing the diameter of the winding core.

The above method may further comprise the step of: after the relaxationstep and before the step of pressing the deformed electrode body intothe flat wound electrode body, rotating the electrode body in thedirection opposite the direction of winding, that is, in a direction ofunwinding the electrode body, the opposite rotation step being carriedout in such a state that the approximately cylindrical electrode body isdeformed into a shape with an oval cross section by pressing theapproximately cylindrical electrode body from a direction perpendicularto the axis of winding.

By carrying out the step of rotating the electrode body in the directionopposite the direction of winding after the step of rotating theelectrode body in the same direction as the direction of winding, therelaxation (clearance) between the electrode boards at the cornerportion of the flat wound electrode body after the pressing processingis enlarged. This further inhibits an increase in the thickness of thecell and deterioration of cycle characteristics.

The number of rotation in the direction opposite the direction ofwinding is preferably from 1 to 3.

In the above method, the relaxation step may comprise: sandwiching theelectrode body between two parallel members for pressing the electrodebody, thereby deforming the electrode body into a shape with an ovalcross section; and with the two parallel members sandwiching theelectrode body, rotating the deformed electrode body in the samedirection as the direction of winding.

In the relaxation step, where the electrode body is deformed into ashape with an oval cross section by being pressed from a directionperpendicular to the axis of winding, if the feature of sandwiching theelectrode body by two parallel members for pressing the electrode bodyis used, it is possible to prepare a flat wound electrode body at lowcost and with ease.

In the above method, the two parallel members may be rotors.

If the two parallel members are rotors, the relaxation step is carriedout with ease.

In the above step, the cross section of the flat wound electrode bodycutting the flat wound electrode body from a direction perpendicular tothe central axis of the flat wound electrode body meets the formulaD1/D2≧1.1, where D1 is the shortest distance of the long axis of thecross section between the inner surface of the innermost layer of theflat wound electrode body and the outer surface of the outermost layerof the flat wound electrode body, and D2 is the shortest distance of theshort axis of the cross section between the inner surface of theinnermost layer of the flat wound electrode body and the outer surfaceof the outermost layer of the flat wound electrode body.

This feature will be described by reference to drawings. FIG. 1 is aview showing a flat wound electrode body of a cell according to thepresent invention. FIG. 1( a) is the cross section of the flat woundelectrode body cutting it from a direction perpendicular the centralaxis of the flat wound electrode body. FIG. 1( b) is a partiallyenlarged view of FIG. 1( a).

In FIG. 1( b), D1 is the distance of the long axis of the flat woundelectrode body between the inner surface A of the innermost layer of thewound electrode body and the outer surface B of the outermost layer ofthe wound electrode body. D2 is the distance of the short axis of theflat wound electrode body between the inner surface A and the outersurface C. An increase in the value of D1/D2 means an increase inrelaxation 1 b (clearance).

If D1/D2 is smaller than 1.1, relaxation 1 b, which is present betweenthe electrode boards at corner portion 1 a, is excessively small,presenting a possibility of bending of the electrode boards. In view ofthis, D1/D2 is preferably 1.1 or larger, and more preferably 1.15 orlarger.

In addition, the secondary cell may be a non-aqueous electrolytesecondary cell, and the negative electrode board may comprise acarbonaceous substance as an active material.

In a non-aqueous electrolyte secondary cell containing a carbonaceoussubstance as an active material, there is a large degree of swelling ofthe electrode boards caused by charging and discharging, and therefore,the advantageous effects of employing the present invention are bestrealized.

(2) In order to accomplish the above and other objects, a secondembodiment of the present invention is configured as follows.

A method for producing a secondary cell having a flat wound electrodebody, the method comprising the steps of; winding, by using a windingcore, a positive electrode board, a negative electrode board, and aseparator provided between the positive and negative electrode boards,and fixing the winding end, thereby preparing an approximatelycylindrical electrode body; after the step of preparing theapproximately cylindrical electrode body, reducing the diameter of thewinding core and rotating the winding core in the same direction as thedirection of winding so as to further wind the electrode boards and theseparator located at an inner layer side of the wound electrode body,thereby relaxing the state of winding; and after the step of relaxation,pressing the electrode body into the flat wound electrode body.

This method includes reducing the diameter of the winding core androtating the winding core in the same direction as the direction ofwinding. By this step, the electrode boards and the separator located atan inner layer side of the wound electrode body are further woundwithout undermining the fixed state of the winding end of the electrodebody. This creates a gap between the inner layer side and outer layerside of the wound electrode body, and thus slightly relaxes the state ofwinding. After this step, by pressing the electrode body into a flatwound electrode body, the relaxation (clearance) is moved to thevicinity of the corner portion of the flat wound electrode body. Asdescribed above with respect to the first embodiment of the presentinvention, this prevents the bending of the electrode boards caused bycharge-discharge cycling. As a result, an increase in the thickness ofthe cell is inhibited and deterioration of cycle characteristics isalleviated.

In addition, since the fixed state of the winding end of the electrodebody is not undermined, the state of facing between the positive andnegative electrode boards is not undermined, either.

In addition, this method is simple in that after preparing anapproximately cylindrical electrode body in a usual manner involvingwinding, the diameter of the winding core is reduced and the windingcore is rotated in the same direction as the direction of winding, andthus excellent in productivity.

In the above method, the relaxation step may comprise fixing anoutermost layer of the wound electrode body, thereby keeping theoutermost layer from being rotated.

If the relaxation step is carried out without fixing the outermost layerof the wound electrode body, it is feared that, following the rotationof the winding core, the outermost layer of the electrode body may berotated in the direction of rotation of the winding core. In view ofthis, the relaxation step is preferably carried out while fixing theoutermost layer of the wound electrode body and keeping the outermostlayer from being rotated.

In the above method, the cross section of the flat wound electrode bodycutting the flat wound electrode body from a direction perpendicular tothe central axis of the flat wound electrode body may meet the formulaD1/D2≧1.1, where D1 is the distance of the long axis of the crosssection delimited by the inner surface of the innermost layer of thewound electrode body and the outer surface of the outermost layer of thewound electrode body, and D2 is the distance of the short axis of thecross section delimited by the inner surface of the innermost layer ofthe wound electrode body and the outer surface of the outermost layer ofthe wound electrode body.

In addition, the secondary cell may be a non-aqueous electrolytesecondary cell, and the negative electrode board may comprise acarbonaceous substance as an active material.

The advantageous effects of these features are the same as thosedescribed in the first embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a flat wound electrode body of a cell accordingto the present invention. FIG. 1( a) is the cross section of this flatwound electrode body and FIG. 1( b) is a partially enlarged view of FIG.1( a).

FIG. 2 is a schematic view of a relaxation processing apparatus used inthe first embodiment of the present invention.

FIG. 3 is a perspective view showing the extent to which the negativeelectrode tab is moved by the relaxation processing.

FIG. 4 is a schematic view showing how the electrode boards of thepresent invention swell.

FIG. 5 is a cross section showing the flat wound electrode body of acell of the present invention after fully charged.

FIG. 6 is a schematic view showing other embodiments of the relaxationprocessing apparatus used in the first embodiment of the presentinvention.

FIG. 7 is a view showing a flat wound electrode body of a cell accordingto comparative example 1. FIG. 7( a) is the cross section of this flatwound electrode body and FIG. 7( b) is a partially enlarged view of FIG.7( a).

FIG. 8 is a schematic view showing how the electrode boards ofcomparative example 1 swell.

FIG. 9 is a cross section showing the flat wound electrode body of acell of comparative example 1 after fully charged.

FIG. 10 is a view showing a flat wound electrode body of a cellaccording to the present invention. FIG. 10( a) is a partial crosssection of example 5 and FIG. 10( b) is a partial cross section ofexample 2.

FIG. 11 is a schematic view of a relaxation processing apparatus used inthe second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described byreference to the drawings in which a non-aqueous electrolyte secondarycell is taken as an example. It should be noted that the presentinvention will not be limited to the following embodiments, and that anymodifications can be made without departing from the scope of theinvention.

(Embodiment 1-1)

Embodiment 1-1 belongs to the first embodiment of the present invention.FIG. 1 is a view showing a flat wound electrode body of a cell accordingto the present invention. FIG. 1( a) is the cross section of this flatwound electrode body and FIG. 1( b) is a partially enlarged view of thecorner portion of this flat wound electrode body.

As shown in FIG. 1, flat wound electrode body 1 of the present inventionhas slight relaxation 1 b (clearance) at corner portion 1 a. The valueof D1/D2 is 1.14.

This non-aqueous electrolyte secondary cell can be prepared by a knownmethod with the use of known materials. For example, as the positiveelectrode material, lithium-containing transition metal compound oxidecan be used such as lithium cobaltate, lithium nickelate, and lithiummanganate. As the negative electrode material, carbonaceous substancesuch as graphite and coke, lithium alloy, and metal oxide can be used.As the non-aqueous solvent, carbonates such as ethylene carbonate anddiethyl carbonate, esters such as γ-butyrolactone, and ethers such as 1,2-dimethoxyethane can be used. As the electrolytic salt, LiN(CF₃SO₂)₂,LiPF₆, and the like can be used alone or in combination of two or moreof the foregoing.

The present invention will be described in greater detail below byreference to examples.

EXAMPLE 1

<Preparation of Electrode Body>

Using a winding core with a round cross section, a positive electrodeboard obtained by a known method, a negative electrode board withnegative electrode tab 2 attached thereto obtained by a known method,and a separator provided between the electrode boards were wound, andthen the winding end was fixed with a tape, thus preparing cylindricalelectrode body 1 (16.5 mm in diameter). After the winding core wasremoved from electrode body 1, as shown in FIG. 2, by using relaxationprocessing apparatus 10 having two rollers 11 placed in parallel,electrode body 1 was pressed to a shape having an oval cross sectionsuch that the short diameter was 15.0 mm (approximately 91% of theoriginal diameter), and electrode body 1 was subjected to two cycles ofrotation in the same direction as the direction of winding (relaxationprocessing). Here the distance (extent of relaxation) of movement ofnegative electrode tab 2 shown in FIG. 3 was 6.2 mm.

After this step, electrode body 1 was pressed from a directionperpendicular to the width direction of negative electrode tab 2,resulting in a flat wound electrode body. Then, the electrode body wasinserted in an outer casing made of aluminum the size of which was 34.6mm long×23.8 mm wide×5.9 mm thick. The outer casing was then impregnatedwith a known electrolytic solution and sealed. Thus, five samples of thenon-aqueous electrolyte secondary cell according to example 1 wereprepared. The average value of D1/D2 was 1.14 (D1/D2 varied between 1.12and 1.18).

COMPARATIVE EXAMPLE 1

Five samples of a non-aqueous electrolyte secondary cell according tocomparative example 1 were prepared in the same manner as in example 1except that no relaxation processing was carried out. The average valueof D1/D2 was 1.05 (D1/D2 varied between 1.01 and 1.07).

<Tests for Increase in Cell Thickness>

Each cell prepared above was charged 0%, 50%, and 100%, and thethickness of each case was measured. The test results are shown in Table1 below. The number of samples subjected to this test was 5 for eachexample.

<Tests for Increase in Cell Thickness Through Cycling>

Each cell prepared above was subjected to charge-discharge cycling underthe conditions shown below. After 500 cycles of charging anddischarging, each cell was charged 0% and 100%, and the thickness ofeach case was measured. The test results are shown in Table 1 below. Thenumber of samples subjected to this test was 5 for each example.

<Conditions for Charge-Discharge Cycling>

Charging: carried out at a constant current of 1 I t (600 mA) to 4.2 V,and then at a constant voltage of 4.2 V for 2.5 hours in total.

Discharging: carried out at a constant current of 1 I t (600 mA) to 2.75V.

<Cycle Characteristics>

After 500 cycles of charging and discharging, the discharging capacityof each cell was measured, and the cycle characteristics were measuredfrom Formula 1 below. The test results (average values) are shown inTable 1. The number of samples subjected to this test was 3 for eachexample.Cycle characteristics(%)=(500−cycle discharge capacity/1−cycle dischargecapacity)×100.  (Formula 1)

TABLE 1 Charge Differ- state Ex. 1 Com. Ex. 1 ence Initial thickness  0%5.93 (5.91-5.96) 5.95 (5.93-5.99) 0.02 (mm)  50% 5.99 (5.97-6.00) 6.02(5.99-6.04) 0.03 100% 6.10 (6.08-6.11) 6.19 (6.17-6.22) 0.09 Thicknessafter  0% 6.33 (6.31-6.35) 6.42 (6.39-6.45) 0.09 cycling (mm) 100% 6.46(6.44-6.47) 6.57 (6.53-6.61) 0.11 Cycle characteristics (%) 90.1 87.22.9

In Table 1 above, the value before each parenthesis indicates an averagevalue, and the values inside each parenthesis indicate variation.

From Table 1, it can be seen that the cells of example 1 are 0.02 to0.11 mm thinner than the cells of comparative example 1.

This can be considered as follows. Since in example 1 the relaxationprocessing was carried out, there was relaxation 1 b (clearance) betweenthe electrode boards at corner portion 1 a of the flat wound electrodebody after being pressed, as shown in FIG. 1. Then, the electrode boardsswelled in the direction that the space was filled. As shown in FIG. 5,the electrode boards therefore did not have any bending after beingcharged. Thus, there was no increase in cell thickness caused bybending.

On the other hand, in comparative example 1, since no relaxationprocessing was carried out, there was no relaxation (clearance) betweenthe electrode boards, as shown in FIG. 7. As shown in FIG. 8, because ofthe tape that fixed the winding end of the electrode body, the directionof swelling of the electrode boards was limited to the inward directionof the electrode body. This caused bending of the electrode boards asshown in FIG. 9. Because of this bending, cell thickness increased.

It can also be seen that the cycle characteristics of example 1 are90.1%, 2.9 percentage points higher than 87.2% for comparative example1.

This can be considered as follows. In example 1, the positive andnegative electrode boards had no bending and were opposed to each otherwithout spaces therebetween, as shown in FIG. 5. There was thereforesmooth progress of charging and discharging, resulting in alleviateddeterioration of cycle characteristics.

On the other hand, in comparative example 1, the electrode boards hadbending as shown in FIG. 9, and there were spaces X at the bent portionsbetween the positive and negative electrode boards. These spaces Xinterrupted the smooth progress of charging and discharging, resultingin increased deterioration of cycle characteristics.

(Embodiment 1-2)

Embodiment 1-2 belongs to the first embodiment of the present inventionand is similar to embodiment 1-1 except that embodiment 1-2 further hasthe step of, after the relaxation step and before the step of pressingthe deformed electrode body into the flat wound electrode body, rotatingthe electrode body in a direction opposite the direction of winding, theopposite rotation step being carried out in such a state that theapproximately cylindrical electrode body is deformed into a shape withan oval cross section by pressing the approximately cylindricalelectrode body from a direction perpendicular to the axis of winding.Detailed descriptions concerning the structure of the cell will not beprovided. This embodiment will be described below by reference toexamples.

EXAMPLE 2

As shown in FIG. 2, by using relaxation processing apparatus 10 havingtwo rollers 11 placed in parallel, cylindrical electrode body 1 preparedin a manner similar to example 1 was pressed to a shape having an ovalcross section such that the short diameter was 16.0 mm (approximately97% of the original diameter), and electrode body 1 was subjected toeight cycles of rotation in the same direction as the direction ofwinding (relaxation processing). Then, electrode body 1 was subjected toone cycle of rotation in the direction opposite the direction ofwinding. Here the distance of movement of negative electrode tab 2 shownin FIG. 3 was 6.9 mm.

Thus, similarly to example 1, five samples of the non-aqueouselectrolyte secondary cell according to example 2 were prepared. Theaverage value of D1/D2 was 1.15 (D1/D2 varied between 1.13 and 1.18).

EXAMPLE 3

As shown in FIG. 2, by using relaxation processing apparatus 10 havingtwo rollers 11 placed in parallel, cylindrical electrode body 1 preparedin a manner similar to example 1 was pressed to a shape having an ovalcross section such that the short diameter was 16.0 mm (approximately97% of the original diameter), and electrode body 1 was subjected tonine cycles of rotation in the same direction as the direction ofwinding (relaxation processing). Then, electrode body 1 was subjected totwo cycles of rotation in the direction opposite the direction ofwinding. Here the distance of movement of negative electrode tab 2 shownin FIG. 3 was 7.0 mm.

Thus, similarly to example 1, five samples of the non-aqueouselectrolyte secondary cell according to example 3 were prepared. Theaverage value of D1/D2 was 1.16 (D1/D2 varied between 1.14 and 1.18).

EXAMPLE 4

As shown in FIG. 2, by using relaxation processing apparatus 10 havingtwo rollers 11 placed in parallel, cylindrical electrode body 1 preparedin a manner similar to example 1 was pressed to a shape having an ovalcross section such that the short diameter was 16.0 mm (approximately97% of the original diameter), and electrode body 1 was subjected to tencycles of rotation in the same direction as the direction of winding(relaxation processing). Then, electrode body 1 was subjected to threecycles of rotation in the direction opposite the direction of winding.Here the distance of movement of negative electrode tab 2 shown in FIG.3 was 6.8 mm.

Thus, similarly to example 1, five samples of the non-aqueouselectrolyte secondary cell according to example 4 were prepared. Theaverage value of D1/D2 was 1.18 (D1/D2 varied between 1.15 and 1.20).

EXAMPLE 5

As shown in FIG. 2, by using relaxation processing apparatus 10 havingtwo rollers 11 placed in parallel, cylindrical electrode body 1 preparedin a manner similar to example 1 was pressed to a shape having an ovalcross section such that the short diameter was 16.0 mm (approximately97% of the original diameter), and electrode body 1 was subjected toseven cycles of rotation in the same direction as the direction ofwinding (relaxation processing). Here electrode body 1 was not subjectedto rotation in the direction opposite the direction of winding. Here thedistance of movement of negative electrode tab 2 shown in FIG. 3 was 6.8mm.

Thus, similarly to example 1, five samples of the non-aqueouselectrolyte secondary cell according to example 5 were prepared. Theaverage value of D1/D2 was 1.13 (D1/D2 varied between 1.11 and 1.17).

<Tests for Increase in Cell Thickness Through Cycling>

Each cell prepared above was subjected to charge-discharge cycling underthe conditions shown below. After 500 cycles of charging anddischarging, each cell was charged 0% and 100%, and the thickness ofeach case was measured. The test results are shown in Table 2 below.

<Conditions for Charge-Discharge Cycling>

Charging: carried out at a constant current of 1.08 I t (650 mA) to 4.2V, and then at a constant voltage of 4.2 V for 2.5 hours in total.

Discharging: carried out at a constant current of 1.08 I t (650 mA) to2.75 V.

<Cycle Characteristics>

After 500 cycles of charging and discharging, the discharging capacityof each cell was measured, and the cycle characteristics were measuredfrom Formula 1 below. The test results (average values) are shown inTable 2. The number of samples subjected to this test was 5 for eachexample.Cycle characteristics(%)=(500−cycle discharge capacity/1−cycle dischargecapacity)×100.  (Formula 1)

TABLE 2 Charge state Ex. 2 Ex. 3 Ex. 4 Ex. 5 Thickness after 0% 6.316.28 6.25 6.35 cycling (mm) (6.28-6.33) (6.26-6.29) (6.24-6.27)(6.32-6.37) 100% 6.46 6.42 6.38 6.51 (6.43-6.48) (6.38-6.44) (6.36-6.40)(6.48-6.53) Cycle characteristics (%) 88% 88% 89% 86%

In Table 2 above, the value before each parenthesis indicates an averagevalue, and the values inside each parenthesis indicate variation.

It can be seen from Table 2 that in examples 2 to 4, which carried outrotation in the opposite direction of the winding direction after therotation in the winding direction, the cell thickness after cycling was0.04 to 0.10 mm thinner and the cycle characteristics were 2 to 3percentage points higher than those in example 5, which belongs toembodiment 1-1 carrying out rotation only in the same direction as thewinding direction.

This can be considered as follows. As described above, by carrying outthe relaxation processing, as shown in FIG. 1, the flat wound electrodebody after being pressed had relaxations 1 b (clearance) at cornerportion 1 a (see FIG. 10( a)). By carrying out the step of rotation inthe opposite direction of the winding direction after the relaxationprocessing, the above relaxations 1 b (clearance) between the electrodeboards were further enlarged (see FIG. 10( b)). This increased theeffect of eliminating the bending of the electrode boards throughcharge-discharge cycling and further reduced the cell thickness,resulting in further improved cycle characteristics.

It should be noted that example 5 had greater cell thickness thanexample 1 and had deteriorated cycle characteristics, which was becauseexample 5 was under more severe conditions for charge-discharge cyclingthan example 1 (1 I t for example 1 and 1.08 I t for example 5).

(Embodiment 2)

Next, embodiment 2 that belongs to the second embodiment of the presentinvention will be described. Because the structure and configuration ofthe cell according to embodiment 2 are similar to those in embodiment1-1, the descriptions thereof will not be provided. Embodiment 2 will bedescribed by reference to examples.

EXAMPLE 6

Using a winding core with a round cross section, a positive electrodeboard obtained by a known method, a negative electrode board withnegative electrode tab 2 attached thereto obtained by a known method,and a separator provided between the electrode boards were wound, andthen the winding end was fixed with a tape, thus preparing cylindricalelectrode body 1 (19.7 mm in diameter) without removing the windingcore.

After this step, as shown in FIG. 11, by using relaxation processingapparatus 20 having two pressing boards 21 placed in parallel, electrodebody 1 with winding core 3 therein was fixed by top and bottom pressingboards 21 for the electrode body as shown in FIG. 11( a), therebyclosing winding core 3 by 0.5 mm (i.e., reducing the diameter of thewinding core by 0.5 mm).

After this step, as shown in FIG. 11( b), with the outermost layer ofthe electrode body being fixed by top and bottom pressing boards 21,winding core 3 was subjected to a quarter cycle of rotation in thedirection of winding to further wind the electrode boards and theseparator located at the inner layer side of electrode body 1, therebyforming a space between the outer layer side and inner layer side of theelectrode body (relaxation processing).

After this step, winding core 3 was removed, and electrode body 1 waspressed from a direction perpendicular to the width direction ofnegative electrode tab 2, resulting in a flat wound electrode body.Then, the electrode body was inserted in an outer casing made ofaluminum the size of which was 35.5 mm long×33.7 mm wide×5.1 mm thick.The outer casing was then impregnated with a known electrolytic solutionand sealed. Thus, a non-aqueous electrolyte secondary cell according toexample 6 was prepared. The average value of D1/D2 was 1.18.

It can be seen from the above that by example 6 it is also possible toform relaxation (clearance) 1 b at corner portion 1 a of the flat woundelectrode body.

This can be considered as follows. Because only the electrode boards andthe separator located at the inner layer side of electrode body 1 arefurther wound without undermining the fixed state of the winding end ofelectrode body 1, a space was formed between the inner layer side andouter layer side of electrode body 1 and the state of winding wasthereby relaxed. After this step, by pressing electrode body 1 into aflat wound electrode body, the relaxation (clearance) is moved to thevicinity of the corner portion of the flat wound electrode body.

It can be seen that similarly to embodiments 1-1 and 1-2, an increase incell thickness caused by charge-discharge cycling is inhibited, anddeterioration of cycle characteristics is inhibited.

(Supplementary Remarks)

The present invention can be applied to cells that use laminate outercasings.

Also, the present invention can be applied to cells that use solidelectrolyte such as polymer electrolyte.

Instead of attaching a negative electrode tab to the negative electrode,a positive tab can be attached to the positive electrode.

The extent of relaxation at the relaxation processing can be adjusted toprevent bending through charge-discharge cycling taking intoconsiderations the thickness of the electrode boards, the number ofwinding, and the amount of change in the thickness of the electrodeboards through charging and discharging.

The amount of deformation by pressing at the relaxation processing ispreferably within the range of from 70 to 97% of the diameter of theapproximately cylindrical electrode body.

In the first embodiment of the present invention, after the electrodebody is wound with a winding core with a space therein as shown in FIG.6, the space is closed to reduce the diameter of the winding core. Then,with the electrode body being pressed into a shape having an oval crosssection with two tension rollers, the winding core is rotated in thedirection of winding and the tension rollers move according to thisrotation. The relaxation processing of the electrode body can be carriedout in the above manner.

In addition, with any secondary cells with a flat wound electrode bodyand electrode boards whose volumes fluctuate through charging anddischarging, sufficient advantageous effects are obtained from theproduction method of the present invention. While in the above exampleslithium-ion cells were prepared, the present invention can be used fornickel cadmium storage cells, nickel hydrogen storage cells, and thelike.

1. A method for producing a secondary cell having a flat wound electrodebody, the method comprising the steps of: winding, by using a windingcore, a positive electrode board, a negative electrode board, and aseparator provided between the positive and negative electrode boards,and fixing a winding end, thereby preparing an approximately cylindricalelectrode body; after the step of preparing the approximatelycylindrical electrode body, reducing a diameter of the winding core androtating the winding core in the same direction as the direction ofwinding so as to further wind the electrode boards and the separatorlocated at an inner layer side of the wound electrode body, therebyrelaxing a state of winding; and after the step of relaxation, pressingthe electrode body into the flat wound electrode body.
 2. The method forproducing a secondary cell having a flat wound electrode body accordingto claim 1, wherein the relaxation step comprises fixing an outermostlayer of the wound electrode body, thereby keeping the outermost layerfrom being rotated.
 3. The method for producing a secondary cell havinga flat wound electrode body according to claim 1, wherein a crosssection of the flat wound electrode body cutting the flat woundelectrode body from a direction perpendicular to a central axis of theflat wound electrode body meets a formula D1/D2≧1.1, where D1 is ashortest distance of a long axis of the cross section between an innersurface of an innermost layer of the flat wound electrode body and anouter surface of an outermost layer of the flat wound electrode body,and D2 is a shortest distance of a short axis of the cross sectionbetween the inner surface of the innermost layer of the flat woundelectrode body and the outer surface of the outermost layer of the flatwound electrode body.
 4. The method for producing a secondary cellhaving a flat wound electrode body according to claim 1, wherein: thesecondary cell is a non-aqueous electrolyte secondary cell; and thenegative electrode board comprises a carbonaceous substance as an activematerial.